Re: [Boost-users] BOOT_PP_REPEAT simple nesting problem

Subject: Re: [Boost-users] BOOT_PP_REPEAT simple nesting problem
From: Colin Fowler (fowler_at_[hidden])
Date: 2014-09-21 14:21:20

On 21-09-2014 17:51, Edward Diener wrote:
<SNIP>
> 
> The BOOST_PP_REPEAT macro takes a single count, so you would need to
> pass into the macro a single repetition count while preserving the
> individual counts in some way for BOOST_PP_REPEAT. The only way you
> could do this is by having the 'data' field of BOOST_PP_REPEAT be more
> complicated, ie. by having the 'data' field include both your 'text'
> and the individual count numbers. When you need a single input to
> include complicated data you should look at how to work with Boost PP
> arrays, lists, tuples, and/or seqs; all of which let you incorporate a
> number of pieces of data in a single input.
> 
> So given:
> 
> decl2 ( int x, 5, 2 )
> 
> and using a Boost PP tuple,
> 
> you want to pass to initially pass to BOOST_PP_REPEAT something like
> (10, DECL2, (text,(5,2))) and then you can work out how to generate
> your output for each of the 10 repetitions of DECL2.
> 
> You can recursively call BOOST_PP_REPEAT from within BOOST_PP_REPEAT
> processing, and while it is advantageous to use the BOOST_PP_REPEAT_z
> form it is not necessary.
> 
> I hope this helps you to figure out how to do what you want.
> 
> 

Hi Edward, that was very helpful, thank you! I had tried using tuples 
yesterday but couldn't get the syntax right. I've come up with the 
following that's very close to doing exactly what I want (I need to 
fiddle around the ordering of the output a bit).

#include <boost/preprocessor.hpp>
#define DECL2B(z,n,text) 
BOOST_PP_CAT(BOOST_PP_CAT(BOOST_PP_CAT(BOOST_PP_TUPLE_ELEM(2, 1, text), 
n), _), BOOST_PP_TUPLE_ELEM(2, 0, text));
#define DECL2A(z,n,text) BOOST_PP_REPEAT(BOOST_PP_TUPLE_ELEM(2, 0, 
text), DECL2B, (n,BOOST_PP_TUPLE_ELEM(2,1,text)))
#define	decl2( text, x, y ) BOOST_PP_REPEAT(y, DECL2A, (x,text))
decl2(int x_, 3, 2)

I have to use BOOST_PP_CAT as for some reason ## still gives surrounding 
spaces after a BOOST_PP_TUPLE_ELEM. Can this be made more readable or am 
I doing things the correct way?

Here's the current output btw: int x_0_0; int x_1_0; int x_2_0; int 
x_0_1; int x_1_1; int x_2_1;

Regards,
         Colin